1. Field of Invention
The present invention relates to a light emitting device control circuit with dimming function and a control method thereof; particularly, it relates to such a light emitting device control circuit which has a headroom regulation function to avoid a flicker of the light emitting devices, and a control method thereof.
2. Description of Related Art
FIG. 1A shows a conventional light emitting device control circuit 10. As shown in FIG. 1A, the light emitting device control circuit 10 receives an AC voltage (AC) and converts it to a DC output voltage Vout which is supplied to a light emitting device circuit 1. The light emitting device circuit 1 for example includes plural light emitting diodes (LEDs). The light emitting device control circuit 10 includes a TRIAC (TRI-electrode AC switch) dimmer circuit 11, a rectifier and filter circuit 13, and a power converter circuit 19, wherein the power converter circuit 19 includes a power stage control circuit 191 and a power stage circuit 192. The TRIAC dimmer circuit 11 receives the AC voltage (AC) (as indicated by a small sinusoidal signal waveform in the figure). When the AC voltage (AC) reaches a predetermined trigger phase, the TRIAC dimmer circuit 11 fires (starts-up), i.e., it is turned ON to generate an AC dimming voltage Vdim (as indicated by a small phase-cut sinusoidal signal waveform in the figure). The rectifier and filter circuit 13 rectifies and filters the AC dimming voltage Vdim to generate an input voltage Vin (as indicated by a small DC signal waveform with ripples in the figure). FIGS. 1B and 1C show schematic waveforms of the AC voltage and AC dimming voltages Vdim1 and Vdim2 with different trigger phases, wherein the AC voltage is indicated by a dash line and the AC dimming voltages Vdim1 and Vdim2 are indicated by solid lines. The rectifier and filter circuit 13 receives the AC dimming voltage Vdim, and rectifies them to generate an input voltages Vin which is inputted to the power converter circuit 19. The power converter circuit 19 is coupled to the rectifier and filter circuit 13, for converting the input voltage Vin to an output voltage Vout according to a control signal ACTL1, and the output voltage Vout is provided to the light emitting device circuit 1. In the aforementioned circuit, the TRIAC dimmer circuit 12 is provided for determining the trigger phase of the AC dimming voltage to adjust an average brightness of the light emitting device circuit 1. The power converter circuit 19 includes a power stage circuit 192 which has at least one power switch. The power stage circuit 192 may be a synchronous or asynchronous buck, boost, or inverting power stage circuit as shown in FIGS. 2A-2F.
To further explain, as an example, let us assume that the power converter circuit 19 includes a buck power stage circuit. FIG. 1B shows that the AC dimming voltage Vdim1 has an earlier trigger phase, and FIG. 1C shows the AC dimming voltage Vdim2 has a later trigger phase. The rectifier and filter circuit 13 rectifies and filters the AC dimming voltages Vdim1 and Vdim2, and correspondingly generates a DC input voltage Vin1 with a higher level and a DC input voltage Vin2 with a lower level Vin2. The control signal ACTL1 is direct proportional to the input voltage Vin (the control signal ACTL1 is a divided voltage of the input voltage Vin), so it indicates the brightness of the light emitting device circuit 1 that a user intends to set. The power stage control circuit 191 controls the power stage circuit 192 according to the control signal ACTL1, such that the conduction time of the power switch in the power stage circuit 192 is controlled according to the level of the input voltage Vin, and the current flowing through the light emitting device circuit 1 is adjusted accordingly, that is, the brightness of the light emitting device circuit 1 is adjusted according to the control signal ACTL1 which is direct proportional to the input voltage Vin.
The aforementioned prior art has the following drawback. Ideally, the output voltage Vout follows the control signal ACTL1 which is direct proportional to the input voltage Vin, so when the input voltage Vin increases, the output voltage Vout also increases, and when the input voltage Vin decreases, the output voltage Vout also decreases. However, in a real case, a power mismatch happens because of various conditions, such as unstable AC voltage frequencies, inconsistent phase-cut angles, etc., and therefore as shown in FIG. 1D, sometimes the input voltage Vin2 may be lower than the output voltage Vout. In this case, the buck power stage circuit can not operate, and a perceivable flicker occurs in the light emitting device circuit 1.
To overcome the drawback of the prior art, the present invention provides a light emitting device control circuit with dimming function and a control method thereof. The present invention provides a headroom regulation function to avoid a flicker of the light emitting device circuit.